Photocurable prepolymers and compositions are well known in the art for forming printing plates and other radiation sensitive articles. In the field of flexographic printing plates, the plates typically comprise a support and a photosensitive layer prepared from the photocurable composition. Additional layers on the plate include slip and release films to protect the photosensitive surface. Prior to processing the plate, the additional layers are removed, and the photosensitive layer is exposed to radiation in an imagewise fashion. The unexposed areas of the layer are then removed in developer baths.
Removal of unexposed layers comprising photocurable compositions such as those disclosed in U.S. Pat. No. 2,760,863 require the use of developer baths comprising environmentally unsafe, organic solvents such as tetrachloroethylene, perchloroethylene, 2-butanone, benzene, toluene, xylene, trichloroethane and solvent mixtures such as tetrachloroethylene/n-butanol. However, due to the toxicity, high volatility and low flash point, their use gives rise to hazardous conditions and creates pollution problems. Thus, recently there has been a strong interest in the field to develop photosensitive layers in non-organic solvent developing solutions, e.g. aqueous or alkaline-aqueous solutions. However, the compositions resulting from recent attempts to achieve aqueous developable plates demonstrate deficiencies in mechanical properties or are unsuitable for use with a wide variety of printing inks, especially alcohol based inks.
For instance, in addition to possessing an aqueous developable photosensitive layer, a flexographic printing plate must have sufficient flexibility to wrap around a printing cylinder, yet be strong enough to withstand the rigors experienced during typical printing processes. Further, the printing plate should possess a low hardness to facilitate ink transfer during printing.
Previous aqueous developable compositions have not possessed all the desirable features such as flexibility, softness and solvent resistance to inks typically used in printing. For example, U.S. Pat. No. 4,023,973 describes a photosensitive composition comprising of a maleic anhydride adduct of a 1,2-polybutadiene. However, because the 1,2 content of this material is very high, i.e. 70% or more, this composition has an undesirably high rubber hardness.
Furthermore, other water-developable photosensitive compositions which contain as the main component a high molecular weight polymer such as polyvinyl alcohol, cellulose, polyethylene oxide, or the like, are insufficient in flexibility and rubber hardness and hence are unsuitable for use in flexographic printing plates.
Finally, it is also important that the photosensitive layer of the printing plate be dimensionally stable during storage. For example, some compositions used for making plates have shown inferior stability properties when used in solid flexographic printing plates in that the compositions become tacky and pasty during storage. Those inferior properties have been attributed to the low molecular weight of the polymers used to prepare the printing plates. See U.S. Pat. No. 4,762,892 to Koch et al. and discussion of low molecular weight polymers disclosed in Japanese Kokoku 57-23693.
One attempt to develop an aqueous developable, flexible printing plate is illustrated in U.S. Pat. No. 4,916,045 (Koch et al., assigned to BASF Aktiengesellschaft). Koch et al.'s '045 patent describes a photosensitive recording element consisting primarily of an ionic polymer. The ionic polymer is prepared by the partial esterification or amidation of succinic acid anhydride side groups on an alkadiene polymer. The resultant polymer is then converted into an ionic polymer by reaction with suitable compounds. With reference to the preferred materials, the reference is directed to oligomers having a polybutadiene backbone, side radicals prepared by (a) maleating the polybutadiene, (b) esterifying the resultant pendant succinic anhydride ring with hydroxyethyl methacrylate (HEMA), and (c) neutralizing the carboxyl groups generated during the esterification with a cation from Groups I to V of the Periodic Table. The preferred cation is zinc.
Two processes are described for the preparation of the partially esterified or amidated polymer. The first process involves the reaction of the succinic acid anhydride side groups with hydroxy or amino derivatives of compounds having an olefinically unsaturated double bond (such as hydroxyethyl methacrylate) to introduce a side radical ("B" in the reference), and, if desired, with compounds containing polar groups to produce a further side radical ("D" in the reference). These two compounds are reacted in one step, not sequentially. The second process for partial esterification involves the hydrolysis of all of the succinic acid anhydride side groups with water and reaction of the acid groups so formed with acid reactive compounds containing unsaturated double bonds to introduce side radical "B" or acid reactive compounds containing polar groups to introduce side radical "D".
However, when prepared from certain monovalent cations, Koch et al.'s ionic polymer does not appear to result in a flexible printing plate having sufficient mechanical properties. For instance, it has been found that when an ionomer prepared according to Koch et al. 's teaching is prepared with sodium, the dimensional stability and flexibility of a printing plate prepared from that ionomer are insufficient for use as a plate in flexographic printing. See Example 8 herein. On the other hand, it has recently been found that monovalent cation, e.g. sodium, based ionomers prepared from other polymers will result in flexographic printing plates having excellent dimensional stability and flexibility. Such plates also demonstrate minimal ink swell in aqueous and solvent based inks. See Example 9.